1. Field of Invention
The present invention relates to a conformal coating for surface mount type electronic ceramic components and to a method of forming insulating coating partially covering the components. More particularly, the present invention relates to the formation of an insulating conformal coating on the surface of ceramic body in the location between each pair of the electrode terminals of the components. Such a conformal coating partially covers the component body and leave conductive electrode terminals uncovered so that the solderability and the electrical conductivity of the electrode terminals are not affected. The partial conformal coating improves the surface insulation resistance of the ceramic components, protects the component from environment, and works as color coding as well.
2. Description of Related Arts
Electronic ceramic components such as ceramic chip capacitors, ceramic chip inductors, ceramic chip resistors, ceramic chip varistors, ceramic chip thermistors, and etc., are widely used in a growing number of electronic products. Although the varied electronic ceramic components are made from different ceramic materials in different electrical configurations, most electronic ceramic components are built according to a common structure as shown in FIG. 1 to comprise a functional ceramic body (11) terminated with two metalized conductive electrode terminals (12) at two opposite ends of the ceramic body.
Following recent miniaturization trends, smaller and smaller electronic ceramic components are created in leadless format to suit surface mount board assembly process for digital and analog circuits, even for high temperature and voltage applications. Surface insulation resistance, one of the most important electrical characteristics of the ceramic component is determined by the component design and surface condition of the ceramic body. Surface insulation resistance can be negatively affected by the ever-shrinking component dimensions demanded by industry, especially for high voltage applications or under serious working environments. A poor surface condition leading to degraded surface insulation resistance can cause many reliability and performance issues, such as:
1) Surface arcing, a corona discharge between the two electrode terminals induced by low surface insulation due to surface contaminations of metal smearing or moisture trapped in the porous ceramic surface, leaves a conductive path on the ceramic body surface leading to component failure.
2) Tin whisker growth, commonly occurring under the influence of complex surface and working conditions when pure tin is plated as the most outside solder layer of the electrode terminals, is another surface condition issue which may cause false data pulses, intermittent failures and, in extreme circumstances, electrical overstress.
3) Difficulties in manufacturing process may also arise from low surface insulation resistance. For instance, certain electronic components made from low insulating ceramics, such as chip inductors made from magnetic ceramics or thermistors made from semiconductive ceramic materials, are hard to be electrical-plated during the production process since metal ions in the plating solution tend to deposit on the surface of the low insulation ceramic body.
To improve the surface condition of an electronic ceramic component, conventionally, a conformal coating is applied to the entire component, covering not only the surface of the ceramic body but the end conductive electrode terminals as well. As early as 1966, Seney et al. disclosed a conformal coating method of applying a glass slurry to the entire surface of a ceramic capacitor to form a thin layer of glaze coverage in U.S. Pat. No. 3,251,918. In 1979, Coleman et al. disclosed in U.S. Pat. No. 4,168,520 a method to attach wire leads to a monolithic ceramic chip capacitor and subsequently coating the leaded capacitor with free-flowed protective resin. In 1986, Muller et al. in U.S. Pat. No. 4,627,139 disclosed a method of dip-coating multiple monolithic ceramic capacitors at one time by pre-positioning the ceramic components on a pair of parallel integral multi-leaded carrier strip to improve the effectiveness of the coating process.
Therefore, it becomes the most conventional method to apply a conformal coating to an electronic ceramic component by attaching lead wires (21) to a leadless component to make it into a wire leaded component then inserting the wire leaded component into a resin solution tank or a resin powder pool to form a fully encapsulated seal (22) of the entire component as shown in FIG. 2, The component is fully encapsulated and the component is capable of being electrically connected through the conductive lead wires.
Such an encapsulated seal does effectively protect the component from environment and enhance the component reliability. However, there are also some negative side effects. In order to keep a conformal coated component electrically connected to the circuit, a metal lead wire (21) or metal lead frame (not shown) has to be attached to each end terminal (12) before the whole component can be conformal coated or encapsulated. The result is a coated lead-attached component that is only suitable for through-hole circuit board assembly process, but not for surface mount assembly process. Besides being procedurally restrictive, the lead attaching process is usually much more costly in terms of material and labor. A heat dissipation problem, especially for high power components, may also arise when a component is fully encapsulated.
For many applications, a full conformal coating where the component body is fully encapsulated may not be necessary. A partial conformal coating applied only to the ceramic body would be commercially acceptable for many applications requiring improvements to the surface condition of the component as shown in the present invention. For example, a ring shaped insulating coating circulating the ceramic body between the terminals can be effective at stopping tin whisker growth. A partial conformal coating covers ceramic surface only would also suppress surface arcing occurrence.